Undesirable excess heat is removed in many industrial processes by the use of heat exchangers in which water is used as the heat exchange fluid. Copper and copper-bearing alloys are often used in the fabrication of such heat exchangers, as well as in other parts in contact with the cooling water, such as pump impellers, stators, and valve parts. The cooling fluid is often erosive and/or corrosive towards these metal parts by virtue of the cooling fluid having high turbidity, aggressive ions, and by the intentional introduction of oxidizing biocides for biological control.
The consequences of such erosion and corrosion are the loss of metal from the equipment, leading to failure or requiring expensive maintenance; creation of insoluble corrosion product films on the heat exchange surfaces, leading to decreased heat transfer and subsequent loss of productivity; and discharge of copper ions, which can then “plate out” on less noble metal surfaces and cause severe galvanic corrosion, a particularly insidious form of corrosion. Also, since copper is a toxic substance, its discharge to the environment is undesirable. Prevention or at least minimization of such discharge is a great problem in view of increasingly stringent public attitudes and legislation relating to pollution of the environment.
It is common practice to introduce corrosion inhibitors into the cooling water. These materials interact with the metal to directly produce a film that is resistant to corrosion, or to indirectly promote formation of protective films by activating the metal surface so as to form stable oxides or other insoluble salts. However, such films are not completely stable, but rather are constantly degrading under the influence of the aggressive conditions in the cooling water. Because of this effect, a constant supply of corrosion-inhibiting substances is generally maintained in the cooling water. A constant depletion of such substances occurs because many cooling systems are open, requiring continuous addition of fresh water to compensate for evaporation and blowdown (i.e., discharge). Continuous addition of fresh corrosion-inhibiting substances is likewise required so as to maintain, within defined limits, a concentration of such substances sufficient for the purpose of maintaining good corrosion inhibition. Moreover, currently used materials do not inhibit erosion of the copper-containing surfaces from the effects of particles in high turbidity water in many industrial processes.
Aromatic triazoles, namely tolyltriazole and benzotriazole, have been used for corrosion protection of yellow metals (e.g., copper and copper alloys) for several decades. However, tolyltriazole is generally preferred because of its lower cost. More recently, butylbenzotriazole and chlorotolyltriazole have also been used in industrial cooling water systems as disclosed, for example, in U.S. Pat. Nos. 4,744,950; 5,772,919 and 5,773,627.
Triazoles function as corrosion inhibitors by adsorbing to copper surfaces, thus providing a protective film that prevents both metal loss and oxygen reduction reactions. However, despite the fact that tolyltriazole and benzotriazole are among the most useful inhibitors for controlling yellow metal corrosion, the performance and cost-effectiveness of triazoles is limited by their consumption in aqueous systems.
The adsorption of triazoles to form protective films results in one form of triazole consumption, but with normal feed rates and metal surface area-to-system volume, this type of triazole loss is typically minimal.
Biodegradation is another known mechanism for the consumption of certain triazoles, such as the 5-methyl isomer of tolyltriazole. Triazoles can also be consumed by precipitation from solution with dissolved copper.
This is not considered a major contributing factor to triazole demand in typical applications, however, where copper is rarely in high enough concentrations to deplete the residual. Another major source of triazole consumption is due to reaction of triazoles with oxidizing halogens.
Many cooling water systems are treated with oxidizing halogens, such as chlorine gas, hypochlorite bleach, iodine/hypoiodous acid, chlorine dioxide, hypobromous acid, bromochloridimethylhydantoin, or stabilized versions of hypochlorous or hypobromous acids, to control microbiological growth. When yellow metals that have previously been protected with triazoles are exposed to an oxidizing halogen, corrosion protection breaks down. Many triazoles, including benzotriazole and tolyltriazole, are vulnerable to halogen attack. Very high dosages of triazoles are frequently added to cooling water systems in an attempt to form new protective films and improve performance.
Not only are triazoles consumed in cooling water systems treated with oxidizing halogens, but the halogens themselves are consumed as well. As the oxidizing halogen attacks the triazole, the halogen is consumed, thereby reducing its biocidal efficiency and reducing cost-performance of the biocide.
Other triazole consumption-related problems associated with combining triazoles and oxidizing halogens in aqueous systems include the formation of (1) volatile by-products which have an objectionable odor and can be released into the environment, (2) by-products that are less effective corrosion inhibitors and (3) toxic halogenated organics. The halogenated organics are particularly undesirable when waters from the aqueous systems are released into the environment, especially into a receiving body of water where toxicity to fish is a concern. Another problem is the inherent aggressiveness of the halogens towards the base metal.
Accordingly, it would be desirable to provide improved compounds, compositions, and methods of inhibiting corrosion of yellow metals in aqueous systems containing oxidizing halogens. It would also be desirable to utilize a corrosion inhibitor resistant to halogen attack and which does not interfere with the biocidal efficacy of the halogen. Furthermore, it would be desirable to provide corrosion inhibitors more environmentally friendly.